COVID-19 Vaccine Allergies? Real But Very Rare. Here’s What You Need to Know

Although there’s a very small risk of severe allergic reactions to the mRNA vaccine, it should not prevent most people – even with other allergies – from being vaccinated.

Vaccine development has never been so quick. We are now at the beginning of 2021 – less than a year after the explosion of the pandemic last March – and we have more than one available COVID-19 vaccine. The first two to roll out are mRNA vaccines from Pfizer/BioNTech and Moderna. On the one hand, that’s terrific news because this will help end the pandemic. On the other hand, none of the phase III clinical trials is complete, and we’re missing critical information on efficacy, effectiveness, and side effects. Among these are COVID-19 vaccine allergies.

The most serious is anaphylactic shock, a reaction that typically occurs seconds or minutes after allergen exposure, as with peanuts, seafood, or bee stings. It can cause a sudden drop in blood pressure, dizziness, a rapid, weak pulse, swelling in the throat or tongue, trouble breathing, hives or skin rash, nausea and vomiting and can be fatal if not treated immediately with an injection of epinephrine.   

But such reactions to previous vaccines are rare – maybe 1 in a million.

mRNA Vaccine Allergic Reactions

Since the Pfizer vaccine rollout, there have been a handful of severe anaphylactic reactions reported in millions of vaccine recipients – not present in the clinical trials, as people with a history of reactions to vaccines had been excluded. So far, there are no reports of allergic reactions to the Moderna vaccine.  

The reaction may be triggered by polyethylene glycol (PEG), which is part of the lipid nanoparticle (LNP) coating the mRNA. However, many other drugs and products like toothpaste, ultrasound gel, shampoo, cosmetics, moisturizers, creams and laxatives contain PEG, contributing to its long history of safe use, and allergic reactions are rare. However, there have been a few such cases involving PEG in bowel preparations for colonoscopy and injectable steroid drugs (methylprednisolone acetate or Depomedrol)). 

But for most people, even those with other allergies to bee venom, medications, or food, or a family history of severe allergic reactions, or even those with a milder, non anaphylactic, allergy to other vaccines, can have the mRNA vaccine.  

To manage possible severe allergic reactions, all healthcare professionals administering the vaccines will follow guidelines similar to those from the U.S. Centers for Disease Control (CDC). They will have equipment for measuring blood pressure and pulse, and emergency drugs such as epinephrine and antihistamines on hand. Also, every recipient must wait 15 minutes after their vaccination, and anyone with risk factors for allergy will require 30 minutes of monitoring at the site. In the case of a severe reaction, the person will need to go to an emergency room or hospital for several hours of monitoring. 

How to Avoid a Severe Allergic Reaction

Although it’s not yet clear what component of the vaccine causes anaphylaxis, here are some tips: 

  • Avoid being vaccinated if you’ve had a severe allergic reaction to any of the ingredients in these vaccines (see table) such as PEG. If you’ve had an allergic reaction or anaphylaxis after preparing for a colonoscopy, after using certain laxatives like Miralax, Colyte®, GoLytely, Colace or other products containing Macrogol. There is a long list of injected PEGylated therapeutics (e.g., Movicol, pegaspargase, pegfilgrastim, PEGylated epoetin beta, PEGylated liposomal doxorubicin, peginterferon alfa‐2a or alfa‐2b, peginesatide, naloxegol, pegloticase, plegridy, certolizumab pegol, pegaptanib, pegvisomant, and pegademase bovine), which could also cause allergic reactions. Skin creams contain a different type of low molecular PEG that seems less likely to cause allergies. If you’re unsure whether your reaction was against an ingredient in the mRNA vaccines, ask your doctor.
  • If you’ve had a severe allergic reaction to the first shot, don’t get the second one. 
  • If you’ve had a severe allergic reaction to other vaccines, ask your doctor if you should get a COVID-19 vaccine and inform the healthcare professional vaccinating you.
  • If you’ve had two doses of one type of vaccine, do not get a second different type of vaccine. 
  • If you’ve had the vaccine and afterwards feel faint, have trouble breathing or any swelling and or tightening in your throat, go back to the place where you got vaccinated, to a hospital or emergency room.

Ingredients in the Currently Available mRNA Vaccines

Nucleoside-modified messenger RNAmRNAmRNA
Lipids (4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2-hexyldecanoate), also called ALC-0315
2 [(polyethylene glycol)-2000]-N,N-ditetradecylacetamide, also called, ALC-0159 [PEG2000]
1,2-dimyristoyl-rac-glycero-3-methoxypolyethylene glycol-2000 [PEG2000-DMG]
1,2-distearoyl-sn-glycero-3-phosphocholine [DSPC]) 
Solution components Potassium chloride
Monobasic potassium phosphate
Sodium chloride
Dibasic sodium phosphate dihydrate
Tromethamine hydrochloride
Acetic acid
Sodium acetate

Source: Food and Drug Administration (FDA) fact sheets 1 and 2

Dr. Michelle Epstein
Michelle Epstein is a medical doctor graduated from the University of Alberta in Canada, who has specialised in Internal Medicine at the University of British Columbia and Allergy and Clinical Immunology at Yale University. Since 2004, she has been a Lab Leader at the Medical University of Vienna’s Division of Immunology.

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